The time division duplex (TDD) in radio communication is a communication method applied to a communication standard such as WiMAX (Worldwide Interoperability for Microwave Access). By switching between transmission and reception time-by-time, full duplex communication is performed in the same frequency band, and thereby efficient use of frequency can be achieved.
FIG. 1 is a diagram illustrating an exemplary configuration of a transmission/reception switch circuit in the TDD radio communication apparatus. An antenna 14 is commonly used for transmission and reception. A circulator 9 is provided for separating a transmission signal from a reception signal. By the above circulator 9, the inflow of a signal from a transmitter unit 12 into a receiver unit 16 is prevented, and also the inflow of a reception signal from antenna 14 into transmitter unit 12 is prevented. By the function of circulator 9, the transmission signal from transmitter unit 12 is transmitted from antenna 14, and the reception signal received by antenna 14 is forwarded to receiver unit 16. A capacitor 8 is a high frequency capacitor for cutting off a direct current component. Diode 6 is provided to prevent the inflow of the transmission signal leaked from circulator 9 into receiver unit 16, and is controlled to turn on/off by means of a controller unit 20. Controller unit 20 controls a switchover between the transmission period and the reception period. Based on preset and fixed switchover timing information, controller unit 20 controls to turn on/off the output of the control signal on the reception side. Namely, in the transmission period, controller unit 20 outputs a control signal on the reception side (supplies a specific voltage), so as to turn on diode 6. As a result, the impedance of diode 6 is made to be substantially 0Ω, and thus, the inflow of the transmission signal leaked from circulator 10 into receiver unit 16 is prevented. On the other hand, in the reception period, by suspending the output of the control signal on the reception side (suspending the supply of the specific voltage), so as to turn off diode 6 and make the impedance of diode 6 infinite. Thus, the reception signal is not made to flow into diode 6, and instead, made to flow into receiver unit 16.
FIG. 2 is a diagram illustrating a switchover state of transmission and reception in a normal case. In a normal state, transmitter unit 12 outputs the transmission signal during the transmission period, while receiver unit 16 receives the reception signal during the reception period. Also, in the transmission period, controller unit 20 outputs the control signal on the reception side. In contrast, in the reception period, controller unit 20 suspends outputting the control signal on the reception side.
In Patent document 1, the official gazette of the Japanese Unexamined Patent Publication No. Hei-8-186551, in case of communication between a single master station and a plurality of substations through the TDMA (Time Division Multiple Access) method, there is disclosed a configuration that each substation outputs the transmission signal during a transmission period successively assigned to each substation. Further, the configuration described below is disclosed: each substation outputs the transmission signal only when a transmission timing signal corresponding to the transmission period is input, and in contrast, when the transmission timing signal is not input, each substation suspends transmitting the transmission signal from a transmission radio section.    [Patent document 1] the official gazette of the Japanese Unexamined Patent Publication No. Hei-8-186551.
In the TDD radio communication apparatus, it is necessary to switch between transmission and reception at high speed. When the output timing of the transmission signal deviates due to an environmental temperature change, an apparatus failure, and the like, there is a risk of the occurrence of a large communication abnormality. More specifically, when the output timing of the transmission signal is delayed from normal output timing, causing the transmission signal output continued in the next reception period subsequent to the completion of the transmission period, the transmission signal flows into receiver unit 16.
FIG. 3 is a diagram illustrating a switchover state of the transmission and the reception in an abnormal case. In the figure, there is shown an abnormal state that the transmission signal is being output in the reception period. Before the completion of the transmission signal output, a shift to a reception period is made. As a result, a portion of the transmission signal flows into receiver unit 16. Here, because the transmission signal power is extremely larger than the reception signal power, and receiver unit 16 is designed to receive a weak radio wave, there is a risk of causing damage or malfunction of circuit elements, such as am amplifier circuit (low noise amplifier, LNA), having a small tolerance of power. Further, because a portion of the transmission signal is not transmitted during the transmission period, the transmission signal power transmitted from the antenna is decreased. This disables transmission of data having integral quality to a radio space.